1. Field of the Invention
The invention relates to the field of industrial ceramics and involves a process for synthesizing aluminum oxides of different crystal structures and the products made from them, such as, for example, can be used as powdered raw materials, as porous filtration membranes or catalyst carriers, as dense sintered substrate layers or dense expendable parts.
2. Discussion of Background Information
Ceramic sintered products based on aluminum oxide (Al2O3), particularly corundum (α-Al2O3), are widely used due to the advantageous chemical and oxidative resistance of these products, in particular in the latter modification. This applies both to dense sintered products (for example, as tool material or wear-resistant machine parts), and to porous components (for example, as catalyst carriers or as filtering material). While dense structures having crystal sizes >2 μm have been known for a long time, it has been possible to produce submicron structures only since the mid-80s by new sol/gel processes and since the beginning of the 90s as a result of the availability of finer crystalline corundum powders (grain size ≧150 nm). Since then, the development of more and more finely structured sintered structures has been a priority goal for ceramic material development, both in the field of dense sintered products with the goal of greater hardness and wear resistance, and in the field of porous materials, e.g., for ultrafiltration membranes. Future advances are determined decisively by further development of more and more fine-grained raw materials.
Synthetic ceramic raw materials can exist in very different forms: in a phase version to be used directly for the subsequent production process of ceramic products (e.g., as corundum [α-Al2O3]), in an intermediate phase (for example, as one of the so-called transitional aluminum oxides, such as γ-Al2O3, which can also be used directly for certain ceramic processes), or as preliminary chemical stages (so-called precursors). Thus, aluminum monohydrates, AlOOH, crystallized as boehmite or diaspore, can serve as precursors for the production of aluminum oxides, while, a stage earlier, compounds such as aluminum chlorohydrate, Al2(OH)5Cl.(2 . . . 3H2O), aluminum sec-butoxide, Al[O(CH3)CHC2H5]3, aluminum tri-isopropoxide, Al[OCH(CH3)2]3, represent inorganic or organic precursors for the production of AlOOH. Most of these raw materials, such as, e.g., boehmite, can exist as a sol, as a dispersion in liquid media, (for example, in water), or as a dry powder and at the same time have different states of agglomeration or aggregation. If the crystallites can occur as coherent aggregates, as agglomerates insoluble with simple agents, or also as separate, easily dispersible individual crystallites, the latter case is desired for many ceramic processing methods. However, with decreasing crystallite size it is increasingly difficult to realize, so that the size of dispersible particles is mostly considerably larger than the crystallite size.
Therefore, for a quantitative, industrially relevant evaluation of the fine grain characteristics of such raw materials, the primary particle size determined preferably by x-ray diffraction (often called crystallite size) must be carefully differentiated from the size of separate particles. The latter is defined by well-dispersed powder suspensions, for example, by means of dynamic light scattering, laser diffraction, or sedimentation (coupled with extinction measurement). Besides the median value of the distribution by volume, D50 (corresponding to 50% by volume), the coarse end of the distribution has particular significance, characterized, for example, by D84, for the technical behavior of the powder. The estimate of average particle sizes from specific surface or from qualitative evaluations of electron microscope photographs, often practiced as a substitute in the lack of adequate powder quantities, can lead to serious estimate errors, in the first case, because the specific surface, as a result of the contributions of surface roughness and finest grain components, is frequently considerably higher than a corresponding average grain size, and in the second case, due to the often insufficient differentiability of coherent crystallites and dispersible particles in the electron-microscopic image. Thus W. M. Zeng et al. (NanoStructured Mater. 10 (1988) 4, 543-550) report of a synthesized “nano” corundum having average crystallite sizes of about 25-30 nm (estimated according to specific surface and TEM), whereas the particle size distribution by volume at d50=0.75 μm has three times the median value of the finest corundum powder commercially available for 10 years.
It is indispensable for the estimation of technically relevant properties (for example, for processibility) to evaluate the mass or volume characteristic property of the particle size distribution, instead of numerical distributions. The latter is often preferred, if exploratory processes yield very small synthesized powder quantities that cannot be evaluated by conventional particle measurement technology, so that distributions are estimated by counting particles using electron microscope photographs. When conversion into a distribution by volume is omitted, the numerical overweighting of the finest grain proportion simulates technically unrealistically low distribution parameters and can cause considerable judgment errors. Thus one publication reports on a so-called “nano” corundum powder having a primary particle size of 50-60 nm (corresponding to the maximum of the numerical distribution curve) and notes that 90% of the particles are smaller than 90 nm (D. Burgard et al., Annual Report of the Institute for New Materials, Saarbrücken, 1996, pages 46-49). The stated number distribution shows, however, that the median value, D50 of the distribution by volume (corresponding to 50% by volume) is 170 nm and D90=300 nm. With that, this “nano” powder is not more fine-grained either than the finest corundum commercially available for the past ten years (e.g., TM-DAR, having D50=140-200 nm, from Boehringer Chemicals, Japan).
The finest grain corundum powders commercially available today at prices <150 DM/kg show average particle sizes ≧150 nm. Further applicable fractions <200 nm having D50=120 nm can be obtained from these at greater cost, but portions <100 nm hardly worth mentioning. Even finer Al2O3 powders are available as so-called transitional aluminum oxides (e.g., γ-Al2O3 or δ-Al2O3), and also layers of such transitional phases also result with the sol/gel synthesis of thin ultrafiltration membranes; still finer pores are produced on the basis of zeolites, TiO2, or mixed oxides. However, these phases have a number of shortcomings regarding both their inherent properties and their behavior in further processing:    In the use of transitional phases of Al2O3 (for example, as a γ-Al2O3 ultrafiltration membrane having pore sizes ≧approx. 3 nm), the chemical and thermal resistance of the products is clearly lower than for corundum. However, it has not been possible to produce the latter in the same grain fineness until now, because the transformation of the transitional aluminum oxides into the α phase occurs only at higher temperatures and then is associated with a coarsening of the particles.    Irrespective of the production route for the synthesis of finer crystalline boehmite starting with commercial boehmite (A. Larbot et al., High Tech. Ceram (1987) 143-151) or by controlled hydrolysis of organic precursors (“Yoldas Process,” S. Alami-Younssi et al., J. Membr. Sci. (1995) Special Issue, 123-129), the finest average pore diameters attainable with Al2O3 membranes (γ-phase) are values between approx. 3 and 5 nm, whereby the boehmite formed as an intermediate stage yields a highly anisotropic particle form of Al2O3, which diminishes permeability by a factor of 2-3 (A. F. M. Leenars et al., J. Membrane Sci. 24 (1985) 245-260). The combination with a hydrothermal treatment makes possible pore sizes of 2.5 nm, if calcination is at so low a temperature that complete conversion into Al2O3 does not occur (M. S. Najjar et al., U.S. Pat. No. 5,139,540: “membrane of . . . alumina containing aluminum-oxygen repeating units and bearing alkoxide groups”). The production of nonmetallic inorganic membranes having pore sizes of less than 1-3 nm has hitherto been described for materials of less chemical resistance, such as hydroxides, zeolites, other silicate compounds and on the basis of TiO2 and ZrO2. In the field of Al2O3 materials, pore sizes between 0.5 and 2.5 nm have been described for “aluminum oxide” compositions of an undefined phase. However, the very special production from aluminoxanes that are difficult to synthesize, extremely reactive and complicated to handle, excludes wider utility (N. v. Thienen, DE 196 38 442 A1: Fluidized Bed Hydrolysis in Inert Gas with Small Quantities of Water Vapor Over Cooled Ice). In the production of dense sintered products starting with transitional aluminum oxides or (in the scope of the sol/gel process) from precursors, there is an unavoidable succession of a whole series of phase transformations that all start with controlled nuclei formation at initially few sites on the still porous structure, and then are spread spherically starting from such nuclei. The spatially heterogeneous distribution of the random nuclei allows the development of irregular, often vermicular pore shapes and crystallite shapes, which adversely affect the properties. Although this undesired structure development in the scope of sol/gel technologies that start from AlOOH, can be largely suppressed by the further addition of the finest possible crystallite corundum nuclei (more recent embodiments, such as U.S. Pat. No. 5,395,407, describe, for example, dense polycrystalline abrasive grains having average crystallite sizes of 0.2-0.4 μm), it has been questioned whether defects typical for the sol/gel process can actually be avoided in this manner (A. Krell et al., EP 678 489 A1).
Therefore, a widespread interest exists in the development of processes that are technically easier to operate for making porous Al2O3 sintered products having pore sizes <2.5 nm (with high permeability) and Al2O3 raw materials that fulfill the condition of particle size <100 nm and are present predominantly as corundum (α-Al2O3). Furthermore, technologies must be developed to convert such raw materials into sintered products. An α-Al2O3 aluminum oxide that meets the cited demands will be designated in the following as nanocorundum for short, and be defined by limiting the parameter D50 to a value <100 nm; advantageously, furthermore, a narrow distribution of particle sizes should also be achieved, described by D84≦150 nm. The difficulty to be overcome in the synthesis of nanocorundum results from the fact that the requirements for extremely fine grain and α-phase (corundum) require opposite synthesis conditions:    the thermodynamically stable corundum phase always requires the highest transformation temperature as the last stage in the sequence of phase conversions,    minimum grain sizes, on the other hand, can be obtained only at minimum temperatures.An advantageous compromise could be reached soonest by using diaspore as the starting material, because this monohydrate, as the single known preliminary stage, converts directly into corundum by avoiding transitional phases and already does so at 450° C. by 750° C. at most. However, diaspore is not known with particle sizes <100 nm from preparation of natural sources or by artificial synthesis (for example, hydrothermally). However, if the starting material is available only as coarse grains, the uniquely low conversion temperature cannot be used to produce nanocorundum, either.
Special routes, such as hydrolysis of activated aluminum layers (Li et al., J. Mater. Sci. Lett. 15(1996)19, 1713) or the exothermic reaction of an aluminum salt (aluminum nitrate) with urea (Bhaduri et al., NanoStructured Materials 7(1996)5, 487) have not hitherto led to adequate quantities of dispersible nanocorundum for tests on the production feasibility of sintered products. Most of the developments pursued until now, therefore, start with nanocrystalline aluminum hydroxides that are convertible to the sol state, preferably boehmite (AlOOH), or with precursors of the hydroxides. In the first case, particular closeness to convertible solutions is given by the fact that companies have long offered commercial boehmite as a powder or sol, its crystallite size being approx. 10 nm and its particle size, depending on the state of dispersion, being described as having values of about 50 nm (for example, from Condea (Hamburg) under the designations Pural SB or Dispersal). Starting with such raw material and without additional measures, such as a nucleating or doping additive, a transformation to α-Al2O3 is observed only from Tα=1205-1220° C.
Within the scope of the use of the cited type of boehmite, it is known that Tα is shifted to lower values by various types of doping or by the addition of nuclei. However, it is also known that a transformation temperature cannot thereby fall appreciably below 1100° C. Thus, according to our own investigations, dopants, such as Ti, Mn, Zr, drop Tα to 1155±20° C., depending on the type of input. Somewhat more favorable values of about 1100±25° C. are attained by the addition of 0.5-4% corundum nuclei or diaspore nuclei having an average particle size of 100-150 nm. An exception among dopants is the addition of ZnF2, which renders possible a transformation temperature of 1035°; however, fluorine promotes grain growth in Al2O3, so that this route is not practicable for the production of finer grain corundum crystallites. Although in principle, we can speculate whether still lower Tα might possibly be conceivable with still finer grain α nuclei <100 nm, in the absence of appropriate investigation possibilities, it has hitherto remained open as to whether the real magnitude of such an effect is actually relevant. However, pertinent nuclei possible for such investigations are not available in defined particle size and purity as long as nanocorundum is unknown (the use of long-known corundum nuclei produced by ball mill grinding is often ruled out for reasons of purity, because really fine grain grinding occurs primarily with Al2O3 grinding balls containing SiO2). Although Burgard et al. claim to have conducted investigations on corundum synthesis from precursors with “nanocrystalline α-Al2O3 seed crystals” (annual report of the Institute for New Materials, Saarbrücken, 1996, pp. 46-49), the stated numerical particle size distribution indicates an average value of 170 nm for the volume distribution of the nuclei and thus hardly differs from the finest grain commercial corundum powder available for 10 years.
Due to these difficulties, there has hitherto been no knowledge indicating a reduction in the transformation temperature Tα of more than 150° C., i.e., to less than 1050° C., starting with boehmite of the aforesaid type, could be expected from the addition of even the finest grain nuclei. Moreover, the above-stated data in the literature on the observed transformation temperatures Tα are related mostly to the temperature of the peaks of DTA curves, recorded at heating rates of 5-10 K/min. This temperature agrees approximately with that at which complete conversion into corundum is reached within a time t<30 minutes. Partial conversion of a lower percent after a longer time is often possible at a lower temperature, but is of little relevance industrially. By annealing for several hours with the addition of the finest grain nuclei, a proportion of 80-100% corundum phase can be achieved from approx. 1000° C. In view of these high transformation temperatures, which have not been fallen below until now, it is not surprising that none of the cited measures has hitherto led to the development of redispersible, commercially feasible nanocorundum.
Therefore, attempts have been made for a long time to overcome the problem of high transformation temperatures and the associated particle growth by synthesizing as a raw material starting from precursors a boehmite that is not only on a nano scale relative to crystallite size, but also dispersible at this level in its particle structure. Conventionally, this takes place by hydrolysis of aluminum alkoxides in water at temperatures >70° C., whereby precipitates are formed that are peptized by the addition of acid (B. E. Yoldas, Bull Am. Ceram. Soc. 54(1975)3, 289-290). The precipitate formation makes it difficult to control particle size as desired, and also the marked shape anisotropy of the resulting boehmite is unfavorable for many uses. However, with extreme effort to suppress the growth of the boehmite crystals (extreme dilution of 0.025 mole al-sec-butoxide, of the sol at a temperature reduced to 8° C.) a beginning corundum formation can actually be observed from 500° C. in the subsequent calcination (approx. 10% corundum after 4 hours—Kamiya et al., J. Ceram. Soc. Japan/Int. Edition 104(1996)7, 664). However, an appreciable yield even of transitional θ-Al2O3 preceding corundum formation starts from 800° C. only (the very clear θ-Al2O3 radiographic diagram from Yu et al., J. Am. Ceram. Soc. 78 (1995) 11, 3149 was erroneously identified as corundum). The actual transformation temperature for corundum formation from optimized organic and inorganic precursors is, according to unanimous data in the advanced literature, at Tα=1100±20° C. and, therefore, not lower than known, also starting with commercial boehmite (doped or mixed with nucleating agents):    1080° C. starting with aluminum tri-isopropoxide (Günay et al., 3 Euro-Ceramics (1993), volume 1, 651),    1100° C. starting with aluminum sec-butoxide or aluminum chlorohydrate (Al2(OH)5Cl.(2 . . . 3H2O) (Oberbach et al., cfi/Ber. DKG 74(1997)11/12, 719),    1095-1113° C. starting with aluminum nitrate (Wood et al., Mater. Res. Symp. Proc. Vol. 180(1990), 97).
In view of the effect of corundum or diaspore nuclei in decreasing Tα by 100-200° C. in the use of commercial boehmite, it was, therefore, to be expected that addition of nuclei in processes starting with precursors would also decrease the corundum formation temperature by a similar amount, i.e., to 900-1000° C. and thus make smaller crystallite sizes possible. However, the results that have hitherto become known from such tests confirm this supposition only with regard to the temperature and only for individual precursors of those investigated. Disappointingly, the desired production of nanocorundum, even where Tα was actually reduced, proved to be impossible by this route and advanced investigations with the finest grain nuclei show, contrary to expectations, a complete lack of influence of the nucleating additives on the grain size of the synthesized corundum:    Starting with aluminum sec-butoxide, 10% by weight Fe2O3 nuclei (hematite) are required to reduce Tα to 970° C., yet the “production of very fine αAl2O3 is not possible by this method” (Oberbach et al., cfi/Ber. DKG 74(1997)11/12, 719).    Starting with aluminum chlorohydrate, although Al2(OH)5Cl.(2 . . . 3H2O), Tα can be reduced even to 750° C., but a crystallite size of approx. 60 nm (at a larger unspecified particle size!) is maintained only if the calcination temperature is limited to 700° C., whereas 900° C. are required for complete elimination of the chlorine which is unfavorable for further use of the synthesis product. A third disadvantage, in addition to particle size and chlorine content, lies in the necessity of using a very special, difficult to produce precursor, Al2(OH)5Cl.(2 . . . 3H2O): if untreated aluminum chloride is used instead, Tα amounts to 1100° C. despite nucleus addition (Oberbach and others, cfi/Ber. DKG 74(1997)11/12, 719). The important role of chlorine for the internal defect structure of the calcined gels and thus for the further transformation behavior was also confirmed by other sources (Wood et al., Mater. Res. Sym. Proc. (1990), volume 180, 97).    Sharma et al. (J. Am. Ceram. Soc. 81 (1998) 10, 2732-34) attempted to solve this problem by the (expensive) route of hydrothermal treatment starting with the customary aluminum chloride, AlCl3.5H2O, and adding 4% α-Al2O3 nuclei. Although it was possible to reduce the temperature required for complete conversion of the preliminary stage into corundum to 950° C., the average particle size increased thereby to 111 nm (compared to 60-70 nm in the hydrothermal product still containing boehmite).    The substitution of the chlorides by aluminum nitrate has not hitherto yielded nanocorundum having a median value of distribution by volume of D50<100 nm, either; surprisingly, additionally added nanocorundum nuclei do not yield more finely crystalline synthesis product. Although DE 195 15 820 A1 (D. Burgard et al.) reports a corundum powder calcined at 1050° C., having a particle size between 40 and 60 nm, the principal author defines these statements more precisely in essential points in two other places. (1) A publication by Ma, Burgard and Naβ (annual report of the Institute for New Materials, Saarbrücken, 1994, pp. 65-67) shows that the information in the unexamined German application concerns a primary crystallite size determined by x-ray diffraction. (2) In an otherwise identical process, finer crystallite size is surprisingly not achieved with the addition of corundum nuclei despite a reduced corundum formation temperature, but rather a similar primary particle size of 50-60 nm is observed, measured at the maximum of the numerical distribution (D. Burgard et al., annual report of the Institute for New Materials, Saarbrücken, 1996, pp. 46-49); the stated numerical distribution curve of the redispersed synthesis product shows that D50 of the distribution by volume lies in the range of 130-170 nm and thus could not be reduced compared to the finest grain commercial aluminum oxides. The nuclei used are described as “nanocrystalline α-Al2O3 produced therefrom [from this synthesis]”, without quantifying the properties in more detail.    α-Al2O3 and hematite nuclei were also used within the scope of the glycothermal synthesis of corundum particles starting with gibbsite (Al(OH)3) dissolved in butanol. Although it was possible to reduce the particle size of the corundum formed by an increasing concentration of α-Al2O3 nuclei from 3-4 βm to ultimately 100-200 nm, the production of corundum was not possible.
According to DE 41 16 523 A1, a process is also known for producing α-Al2O3 through hydrolytic condensation of aluminum compounds. Accordingly, aluminum compounds that can be hydrolyzed are converted with β-diketones into aluminum/β-diketone complex compounds before hydrolysis, subsequently these compounds are condensed and heated to relatively high temperatures >900° C., e.g. 1100° C.
Likewise known is a process according to WO 95/12547 for producing water dispersible aluminum hydrates of boehmite structure and application of the same. Colloidal dispersions with a pH value of between 3 and 7 are produced in which the boehmite or pseudoboehmite is present in nanocrystalline structures. A corundum polishing medium produced from this shows crystallite sizes of 60-90 nm determined by x-ray diffraction; information on particle size was not given.
According to EP 0 554 908 A1, fine SiO2-coated α-aluminum oxide powders, their production and use are known. This nano-powder of α-aluminum oxide can be produced from boehmite gel that is coated with a barrier-forming material, such as silicon oxide, by drying and sintering. However, pure Al2O3 powders cannot be produced by using a grain growth inhibitor (SiO2).
In summary, the disadvantages of the known prior art can be described in that no process exists with which a chlorine-free nanocorundum having a particle size distribution described by D50<100 nm can be produced with or without the use of nuclei in a quantity adequate for further processing into sintered products; the expression chlorine-free describes here compositions having less than 0.05% by weight chlorine (e.g., as an impurity). Also, neither sintered corundum products with submicrometer or nanostructures, which can be produced from such nanocorundum, nor nanoporous Al2O3 sintered products having pore sizes in the range between 0.5 and 2.5 nm that can be produced from raw materials easy to handle are known.